1. Field of the Invention
The present invention relates in general to a method for constructing absorber towers, and in particular, to a method for erecting an absorber tower for flue gas desulfurization (FGD) using a jacking system.
2. Description of the Related Art
Absorber towers are devices known in the art, and are employed in conjunction with furnaces or boilers, as part of their flue gas desulfurization (FGD) system. The purpose of the flue gas desulfurization system is to treat the flue gas emissions produced by the combustion process taking place in the boiler.
When planning the installation of a new boiler or furnace such as for a modern utility power plant, absorber towers are often included in the overall scope of work for the project. However, many of the existing boilers in use today were not originally equipped with absorber towers, and in fact are operating with no means provided for flue gas desulfurization.
The recent enactment of the Clean Air Act requires utilities and industry to limit their operations' flue gas emissions, so as to be at or below specified compliance limits. As such, viable options for minimizing said emissions are being sought and implemented. The installation of flue gas desulfurization systems, with their respective absorber towers, is one means of ensuring compliance with the Act.
In the case of an existing plate site, the installation of absorber towers must be performed on a retrofit basis. Space available for (1) material receipt, storage, laydown, and staging; (2) ground assembly of FGD system components; and (3) construction accessibility, is typically limited on these types of installations. This space limitation presents a problem to the FGD system owner and erecting contractor(s) with regard to work scheduling, logistics, and overall productivity.
To date, several scenarios of absorber tower shipping configuration/erection method have been realized. One scenario has been to maximize absorber tower shop fabrication and assembly, and ship a minimal number of "modules" per absorber tower to the jobsite. A typical "module" has consisted of a circumferential shell complete between established horizontal field weld lines, with external stiffeners, internal supports, and respective absorber internals installed. Upon receipt on the jobsite, modules have been "stacked" on top of each other, horizontal field welds completed at the splice lines, and upon completion of field testing, the absorber tower was ready for operation. This scenario is an effective approach contingent on the existence of the following conditions:
1. A jobsite accessible via a navigable waterway; PA1 2. An absorber tower fabricator with facilities, material handling, and barge loading capabilities on a navigable waterway; PA1 3. Barge landing and off loading facilities available on the jobsite; PA1 4. Jobsite accessibility for transport of the modules from the barge landing and off loading area to the point of final absorber tower installation; PA1 5. Available space on the jobsite for the placement and utilization of heavy lift cranes for the erection of absorber tower modules in their final position.
Although this approach has proven to be effective on certain projects in the past, the jobsite enhanced by each of the above conditions in rare.
In the absence of a navigable waterway, or when the jobsite is not conducive to the receipt of shop assembled modules, absorber tower material has been shipped to the jobsite in a "knocked down" configuration. Shell plates have been provided in sizes commercially available from the mills, typically 8'.times.20', shop rolled to the curvature of the respective absorber tower shell, and delivered to the jobsite in specially designed cradles, either via trunk or rail load. External stiffeners, internal support members, and absorber internals have been shipped as loose pieces for field installation. Upon receipt of the loose material on the jobsite, two basic methods have been used for the erection of the absorber tower. Given the availability of space for ground assembly "tables" in close proximity to the final location of the absorber tower, loose shell plates have been fit and welded as required to form continuous shell course "rings". Depending on available crane capacity and the accessibility from the ground assembly table to the final location of the absorber tower, shell course "rings" may have been further ground assembled and welded two or three high on the table. Loose stiffeners, internal supports, and absorber internals may have been installed on the ground assembly table as well. Upon completion of the ground assembly activity, the effort for final erection of the absorber tower in place became similar to that required for the erection of shop assembled modules. Heavy lift cranes have been used to "stack" the ground assembled shell courses on top of each other, so as to allow for completion of the horizontal weld between them.
If space has not been available on the jobsite for a ground assembly area, absorber tower components received "knocked down" have been erected, fit, and welded piece by piece in place. The absorber tower was scaffolded as required to access the work, and crawler cranes or derricks were provided for handling the loose material from the ground to final position in the absorber tower. Further, until such time as the tower is inherently structurally stable, temporary bracing, supports, and shoring have been provided as required to withstand the effects of wind and construction dead loads encountered during the erection process.
Associated with each of these absorber tower shipping configuration/erection method scenarios has been a unique set of costs, benefits, advantages, disadvantages, and required conditions for their implementation. In the case of shop assembled modules, benefit has been derived in minimizing the amount of field labor and time required for the erection of an absorber tower. This savings in field labor and time has been offset by the increased costs of transporting and handling heavy modules from the shop to the towers' final location on the jobsite. On the other hand, shop, transportation, and lifting equipment costs have been minimized with the provision of "knocked down" material; but costs associated with increased field labor, schedule time, scaffolding, and the achievement of a quality product have tended to make this option unattractive to the absorber tower erecting contractor. Nevertheless, the option finally selected for a particular project is governed by a unique set of site specific conditions.
Retrofit installations typically present the worst possible conditions to be faced by the absorber tower erecting contractor. In most cases, they are not accessible via navigable waterway; jobsite access and space availability is minimal; and the project construction time span is accelerated to beat a scheduled FGD compliance date. Hence, there is a need for a method of erecting absorber towers in retrofit applications with minimal access and available space. The method should preclude the need for heavy construction equipment, and should minimize the amount of scaffolding required to access the work. The method should be adaptable to any jobsite, regardless of its location and specific site conditions.